The present invention relates to an apparatus for mounting twin annular diameter blades and in particular to a hydraulically operated blade tensioning means.
The use of extremely thin semiconductor wafers, for example, silicon wafers in the Electronics industry, typically in the order of about 0.010 inches in thickness, has been on the increase in recent years. Such wafers are usually cut from large semiconductor crystal rods drawn specifically for the purpose of providing material to be cut into these individual wafers.
The diameter of the specifically-drawn rods are approximately 3 inches or more. Uniformity of size and surface configuration of the sawed wafers is extremely important. Semiconductor materials are usually prepared by crystal growth techniques in the shape of rods. These rods must be cut into the thin wafers which then serve as substrate for semiconductor device processing. The cutting is usually accomplished by annular saws featuring a thin inside diameter blade with diamond deposits upon the cutting edge of the inner opening of the blade. These special cutting blades have been devised in order to slice thin wafer material from large diameter rods while minimizing waste of the expensive semiconductor crystal material and providing wafers which have smooth, unscarred surfaces. The achievement and maintenance of the proper tensioning of the inside diameter blade is considered a major and serious problem.
The cutting blades must be thin enough to provide a desired thin wafer while minimizing waste of material. The cutting blades must also have the rigidity to be able to slice through a 3 inch or greater diameter rod. These requirements demand a blade with the thinness and rigidity not obtainable in ordinary outside diameter cutting blades. In order to provide a cutting blade with the required characteristics of thinness, rigidity and cutting depth capability, a thin circular or annular saw blade is clamped about its circumference and an opening is provided in the center of the blade. The blade so formed is then mounted on a saw blade housing, thus forming an inside diameter cutting surface.
In earlier slicing machines mechanical means were provided for tensioning the saw blade once it was clamped. Mechanical tensioning, however, proved to be less than satisfactory as it was difficult, if not impossible, to control or predict the amount of tension which would be transimitted to various circumferential locations of the saw blade. The difficulty arose as a result of the difficult precise repeatable movement of mechanical tensioning means at specific fixed positions about the saw blade's peripheral. Further, due to the mechanical application of tensioning forces, the mechanically tensioned saw blades experienced very severe hysteresis and frictional losses. These losses contributed to the inaccuracy of the mechanical tensioning system.
More recently hydraulic blade tensioning systems have appeared on the market. In some of the systems available, the hydraulic fluid or grease which is used as the tensioning medium is contained in an annular groove in a clamping ring of the slicing machine. The grease is pressurized and forced into direct contact with a segment of the saw blade near the perhipery thereof. The pressure which is applied to the tensioning medium, in turn, impinges upon the blade and gives a significantly more predictable amount of tension for a given amount of hydraulic pressure than was possible with mechanically tensioned saw blades.
In known clamping devices, axial pressure deforms the saw blade either through hydraulic or mechanical devices. However, such devices have been considered with only one blade on the sawing head. Tension control of these single inide diameter sawing blades have been sufficiently complicated that only single blades have been attempted by the industry. Indeed, other cutting technology improvements have been considered such as enlarging the center hole of the blade because the material to be cut has been increased in diameter, and trying to speed up the cutting rate to improve productivity. These attempts have not all necessarily been successful and mutliblade cutting as a means to increase productivity has not been considered because of inherent tension control problems of single inside diameter blade sawing.
It is an object of the present invention to provide an apparatus for twin blade mounting and tensioning through pressurized fluid expansion chamber means.
Another object of the present invention is to provide a means for uniformly tensioning twin or multiple blades about their periphery and also to enable the amount of tension to be rapidly changed during continued operation, without requiring the assembly to be removed from annular clamping - tensioning members. Consequently, the present invention provides an apparatus which permits twin inside diameter saws to be retensioned during continued use and, therefore, extends the useful life of the blades while providing increased sawing productivity from, presently, in use inside diameter saw machinery.